This invention relates generally to automatic guided vehicles, and more particularly to the steering and control of automatic guided vehicles. Automatic guided vehicles, often referred to as AGVs, are driverless vehicles that are often used for material handling purposes. AGVs are capable of carrying or towing material from one point to another without the need for a driver. AGVs generally come in two types, depending upon how they guide themselves. In a first type, the AGVs guide themselves by following current-carrying wires buried in the floor. Such AGVs typically have sensors positioned on their underside which are able to detect the magnetic field created by the current flowing through the wires. By laying these wires along desired pathways, the AGV is able to follow the wires to its intended destination, thereby avoiding the need for a human to steer the vehicle.
A second type of AGV guides without the use of wires, and is generally referred to as a wireless AGV. These AGVs are capable of driving themselves from a first location to a second location without the need of wires imbedded in the floor. Instead of following the wires, the wireless AGVs use navigation sensors to determine their position and heading. This position and heading information is then used by the vehicle in order for it to automatically steer itself along a desired path. The navigation sensors may include gyroscopes, sensors for detecting magnets embedded in the floor, laser reflectors, wheel encoders, transponder sensors, and a variety of other types of sensors.
Whether of a wire or wireless type, prior art AGVs have typically steered themselves to desired locations by first determining their position, comparing this position to a desired position, and implementing an appropriate steer correction based upon the difference between the desired and measured position. The AGV repeats this process as it moves. For tricycle style AGVs that include a front steered wheel and two rear, unsteered wheels, the steering correction is applied to the front, steered wheel. For AGVs that use differential steering (steering by running side-by-side wheels at different velocities), the steering correction is translated into appropriate velocity commands for each of the side-by-side wheels and applied to them. In the past, AGVs which have guided themselves by this method have suffered from the potential to increase their heading errors while making corrections to their position. This is due to the fact the AGV can only attempt to correct its position error by making changes in its heading. Oftentimes this change in the heading creates an even larger heading error.
An example of this heading error magnification is depicted in FIG. 5. An AGV 100 is depicted in an initial position 102a in FIG. 5. In position 102a, vehicle 100 is oriented parallel to a guidepath 104, and thus has no heading error. Vehicle 100 includes a center guidepoint 106 which denotes the point on the vehicle which the vehicle considers to be its position. Stated alternatively, guidepoint 106 is the point on the vehicle which the vehicle attempts to maintain over guidepath 104. Therefore, in initial position 102a, vehicle 100 is laterally offset to the left of guidepath 104. In response to this position error, vehicle 100 would turn its wheels to the right to thereby steer back toward guidepath 104. As illustrated in positions 102b, c, d, e, and f, the steering of vehicle 100 back toward guidepath 104 will cause vehicle 100 to change its orientation. In position 102b, vehicle 100 has rotated several degrees in a clockwise direction and is no longer oriented parallel to guidepath 104. Vehicle 100 therefore has gone from position 102a, in which it had no heading error (i.e., it was parallel to guidepath 104), to position 102b, in which its heading is different from the orientation of guidepath 104. In position 102c, vehicle 100 has rotated even further in a clockwise direction, thus increasing its heading error with respect to guidepath 104 even further. Thus, the correction of the position error of vehicle 100 in initial position 102a is only corrected by increasing the heading error of vehicle 100.
There are several disadvantages resulting from the AGV control scheme illustrated in FIG. 5. First, as can be seen in FIG. 5, the rotation of vehicle 100 increases the necessary width of the corridor down which vehicle 100 travels. Therefore using AGVs which steer as illustrated in FIG. 5 require corridors of sufficient width to accommodate the rotation of the AGV as it steers itself along the guidepath. Additionally, AGVs that guide in the manner illustrated in FIG. 5 often have severe heading errors after they have guided around a curved or arced portion of a guidepath. As the vehicle completes the turn, it often has a significant heading error that only decreases after a significant amount of straight guidepath has been traversed. This is illustrated in FIG. 7 wherein an AGV 100 includes a front steered wheel 108 and an unsteered rear wheel 110 (as well as a suitable number of support casters which are not illustrated). The point above front wheel 108 is assumed to be the guidepoint 106, and vehicle 100 is illustrated in four different positions in which guidepoint 106 is perfectly aligned with a guidepath 104 (i.e. no position error). As can be seen, when vehicle 100 reaches position 102d, it is substantially misaligned with guidepath 104. Thus, it is virtually impossible to have vehicle 100 stop immediately after this turn and be oriented in the same direction as guidepath 104. These and other disadvantages arise from prior art methods of steering and controlling the movement of AGVs. The desire for an AGV control method that overcomes these disadvantages can therefore be seen.
Accordingly, the present invention provides a method for controlling an automatic guided vehicle which overcomes these and other disadvantages of prior art guidance methods. The present invention not only allows for AGV corridors to be narrower, but it more accurately controls the heading of AGVs as they traverse turns. The present invention provides the AGVs with a method of independently simultaneously being able to control both their heading and position.
According to one aspect of the present invention, a method for controlling an automatic guided vehicle includes measuring the AGV""s heading and location. Any error between the AGV""s measured heading and desired heading is determined. Also, any error between the AGV""s measured location and a desired location is determined. The vehicle is then steered to simultaneously attempt to reduce both the error between the AGV""s measured heading and desired heading and also the error between the AGV""s measured position and the desired position.
According to another aspect of the present invention, a method for controlling an automatic guided vehicle includes measuring the AGV""s heading and location, along with providing a first control loop that generates a steering command based upon any difference between the measured AGV heading and a desired AGV heading. A second control loop is also provided that generates a steering command based upon any difference between the measured AGV position and a desired AGV position.
According to yet another aspect of the present invention, a method for controlling an automatic guided vehicle is provided. The method includes measuring the AGV""s position and determining a desired position for the AGV. The AGV""s measured position and desired position are compared and a steering command is generated for the AGV based upon any difference between the measured position of the AGV and the desired position of the AGV. The steering command alters the AGV""s position without altering the AGV""s orientation as the AGV moves.
According to still another aspect of the present invention, an apparatus is provided for controlling an AGV. The apparatus includes a first and a second controller, each of which may be implemented in separate hardware modules or resident as separate control equations resident in a single processor. The first controller determines any difference between the vehicle""s measured position and a desired position and outputs a command that tends to reduce any such difference. The second controller determines any difference between the vehicle""s measured heading and a desired heading and outputs a command that tends to reduce any such difference.
In other aspects of the invention, the AGV includes first and second sets of wheels that are spaced apart longitudinally on the vehicle. A steering command adapted to correct the vehicle""s position is applied to the first set of wheels. A steering command adapted to correct the vehicle""s heading is added to the steering command adapted to correct the vehicle""s position and the sum is applied to the second set of wheels. The desired heading and desired position may both be derived from a guidepath stored or created onboard the AGV.
The present invention enables an AGV to simultaneously control both its heading and position. Thus, heading errors can be corrected without significantly affecting the vehicle""s position error, if any. Similarly, any error in the vehicle""s position can also be corrected without significantly affecting the vehicle""s heading. This type of control allows a vehicle to take tighter turns, more accurately control heading, and move down narrower corridors. These and other benefits, results, and objects of the present invention will be apparent to one skilled in the art, in light of the following specification when read in conjunction with the accompanying drawings.